Annexins became attractive therapeutic targets for the treatment of various diseases, including cancer, cardiovascular diseases and diabetes, because of their ability to modulate the activities of important signaling pathways involved in disease pathology. Annexin (Anx) A5 and AnxA6 are highly expressed in osteoarthritic (OA) cartilage; however, nothing is known about their role in OA pathology. Two major signaling pathways with roles in OA are NF-?B and the Wnt/-catenin (Wnt), encouraging us to examine the potential interaction between AnxA5 and AnxA6 and these signaling pathways. NF-?B signaling pathway, which is being activated in articular chondrocytes by various cytokines, including interleukin (IL)-1, is one of the major catabolic signaling pathways in OA, whereas the Wnt signaling pathway has been recently shown to act anti-catabolically in human articular chondrocytes while acting catabolically in mouse chondrocytes. Our preliminary findings show that AnxA5 and AnxA6 stimulate NF-?B signaling while inhibiting Wnt signaling. In addition, our findings suggest that AnxA5 modulates NF-?B and Wnt signaling via direct interaction with calpastatin, an inhibitor of the cysteine protease calpain, thereby stimulating calpain activity. Calpain stimulates NF-?B signaling, while inhibiting Wnt signaling. AnxA6, on the other hand, stimulates NF-?B signaling via direct binding to the p65 unit of the NF-?B complex, while it inhibits Wnt signaling via interfering with membrane association of the Wnt signaling complex, which is required for Wnt signaling activation. Based on these findings, we hypothesize that AnxA5 and AnxA6 act via different mechanisms, to modulate NF-?B and Wnt signaling to stimulate cartilage destruction during OA pathology. To test our hypothesis, we are proposing two aims. In the first aim we will determine the nature of the AnxA5/calpastatin and AnxA6/p65 interactions, and how these interactions affect NF-?B signaling activity. In addition, we will determine how the AnxA5/calpastatin interaction affect Wnt signaling and how Ca2+-dependent plasma membrane association of AnxA6 interferes with the membrane association of the Wnt signaling complex and ultimately Wnt signaling activity. Furthermore, we will determine how AnxA5 and AnxA6 individually and together affect NF-?B and Wnt signaling during aging, IL-1 injection in the knee joint or surgically induced OA in AnxA5 and AnxA6 single and double knockout mice. In Aim 2, we will determine the effect of annexin-mediated modulation of these signaling pathways on the function and phenotype of human articular chondrocytes. Finally, we will determine how annexin-mediated inhibition of Wnt signaling affects OPG expression in AnxA5 and AnxA6 single and double knockout articular chondrocytes and AnxA5 and AnxA6 overexpressing chondrocytes and ultimately osteoclastogenesis and subchondral bone changes in OA pathology. We expect that the successful completion of this proposal will provide novel mechanisms stimulating cartilage destruction during OA pathology and novel therapeutic targets for the treatment of OA.